The present invention relates to a liquid crystal display device in which a lighting unit such as a backlighting unit is employed.
In recent years, implementation of personal computers inclusive of so-called word processors in a small size has been promoted, and portable type personal computers known as lap-top type or notebook type computers are widely used. In such portable type personal computer, a liquid crystal device is commonly used as a display unit. In this conjunction, there is an increasing tendency for adopting color display in the portable type personal computers. In accompanying with such trend, a backlighting type liquid crystal display device is coming into wide use, in which a light source is disposed at a rear side of a liquid crystal display screen for lighting the whole display screen or panel from the rear or back side. Needless to say, the backlighting light source for the color liquid crystal display device is required to emit light with high luminance or intensity. Besides, it is necessary to illuminate the display screen with uniform luminance over the whole planar surface thereof. Luminance of the backlighting can easily be increased by increasing that of the light source. However, taking into consideration the fact that the portable-type personal computer or word processor or the like are usually operated by using a battery or cell, limitation is unavoidably imposed to the attempt for increasing the luminance of the light source. To say in another way, there has been proposed no effective method or measures for increasing the luminance of the liquid crystal display screen.
For having better understanding of the present invention, conventional liquid crystal display devices such as disclosed, for example, in Japanese Unexamined, Patent Application Publications Nos. 162002/1992 (JP-A-4-162002) and 67004/1994 (JP-A-6-67004), will first be reviewed briefly. FIG. 33 shows a lateral source type backlighting device employed conventionally in the liquid crystal display device known heretofore. Referring to the figure, a lamp such as a cold-cathode discharge tube or a hot-cathode discharge tube is employed as a light source 1 which is disposed at and along one lateral side of a light guide plate (also known as optical waveguide plate) 2 which is made of a light-transmissive material, wherein a light scattering layer 3 for scattering light rays and a light reflecting sheet 4 for reflecting light rays are disposed on the bottom surface of the light guide plate 2 while a diffusing sheet 5 formed of a synthetic resin of milk-white color having a light scattering effect is mounted over a top surface of the light guide plate 2 with a view to uniformizing luminance of the backlight over the whole display screen. Additionally, there are disposed on the diffusing sheet 5 a pair of light collecting plates 6 and 7 in order to enhance axial luminance (i.e., luminance in the direction orthogonal to the display screen) of the liquid crystal display device by converging diffused light rays.
On the other hand, according to another proposal disclosed, for example, in Japanese Unexamined Patent Application Publication No. 294745/1995 (JP-A-7-294745), grating grooves are formed in the bottom surface of the light guide plate for reflecting the light rays incident on the light guide plate.
As is apparent from the foregoing description, in the conventional backlighting units for the liquid crystal display devices known heretofore, light rays emitted from the light source 1 and introduced into the optical waveguide or light guide plate 2 undergo scattering at light scattering substance contained in the light scattering layer to illuminate the liquid crystal element after having transmitted through the diffusing sheet. It can readily be understood that the structure of the conventional backlighting optical waveguide or light guide panel for the liquid crystal display device is much complicated. Besides, the luminance of the liquid crystal display device becomes lower as a whole because of loss due to the light scattering, giving rise to a problem. Further, the backlighting unit provided with the grating grooves is difficult to manufacture by resorting to a die molding method or the like.
As an approach for decreasing the number of parts of the backlighting unit as well as the number of steps involved in assembling or manufacturing the same while enhancing the performance or operation characteristics thereof, it is conceived to develop a novel structure composed of concave and or convex portions effective for reflection, scattering, refraction, diffraction, absorption, transmission, polarization or deflection of light rays or portions exhibiting variable optical characteristics or portions imparted with optical activity such as electro-optic effect. Parenthetically, the concave portions, the convex portions, the portion having variable optical characteristics and the optically activated portion mentioned above will hereinafter be referred to generally as the dot. By forming the dots mentioned above on a top or bottom surface of the light guide plate or alternatively on an optical member disposed on the top or bottom surface of the light guide plate, the diffusing sheet 5 and the light collecting plate 6 can be spared.
However, the structure described above suffers a problem that when the dot area is dimensioned large, there takes place a so-called dot visibility phenomenon, i.e., the phenomenon that the dots are visually perceived as spot-like objects. This problem can be coped with by deceasing the dot area so that the dots can not be visually perceived. In that case, however, because the dots are disposed regularly, there may arise another problem that moire takes place due to interference between the dot array and an RGB-matrix of a color filter provided on the top or bottom side of the optical member disposed on the top or bottom surface of the light guide plate or a regular pattern of thin film transistors (TFTs).
On the other hand, when the dots are disposed at random in an effort to suppress the moire phenomenon, then nonuniformity or unevenness in the luminance distribution as well as the dot visibility problem mentioned above may be brought about due to unevenness in the dot density, giving rise to yet another problem. For dealing with these undesirable phenomena, use of a diffusing sheet may be conceived. However, incorporation of such diffusing sheet in the liquid crystal display device in turn means that the number of constituent parts of the device increases while presenting a cause for degrading the transmittance, impairing eventually brightness of the display screen.